1. Field of the Invention
The invention is in the field of sputter coating processes.
2. Brief Description of the Prior Art
Sputtering is a thin film coating process which makes use of a plasma discharge in a sputtering gas to bombard a body of the material to be sputtered (sometimes known as the "target"). Because of the ion bombardment, neutral atoms of the sputtered material are ejected from the target surface and are deposited on the substrate to be coated. In order to sustain the plasma discharge a certain minimum gas pressure is required in the closed sputtering chamber. For several reasons it is often desirable to maintain the pressure of the sputtered gas at as low a value as possible. A class of sputtering sources has been developed which permits the use of relatively low gas pressures. (See, for example, U.S. Pat. No. 3,616,450 issued Oct. 26, 1971; and U.S. Pat. No. 3,711,398 issued Jan. 16, 1973.) This source employs, among other things, a magnetic field in the plasma region, to increase the path length of electrons, thus increasing the probability of an ionizing collision. This permits a plasma to be sustained in lower gas pressures than otherwise possible. However, using such a source the substrate to be sputter coated, which must be situated relatively close to the source, is in the fringing field of the source magnet. This fringing field has the subsidiary effect of guiding some charged particles (principally electrons) to the substrate, producing an incident charged particle flux. This charge particle flux produces heating of the substrate over and above the substrate heating which is inherent in the sputtering process, due to the flux of neutral particles of the sputtered material. In some situations this heating may be considered beneficial, for example, where it provides in situ annealing of the sputtered film. However, in some situations such heating may be detrimental, for example, if the substrate is degraded by heating. An example of the latter is the deposition of metals on certain polymer films.